In general, the sintered body made by powder metallurgy is advantageous in cost over ingot steels obtained through forging and rolling steps and has wide utility as parts of motor vehicles and office automation apparatus. However, the sintered body has voids which are inevitably formed during the course of its fabrication, thus leading to the drawback that strength, fatigue strength and toughness are low. In order to enlarge the range in use of the sintered body, it is important to improve the strength, fatigue strength and toughness.
In order to improve the strength of sintered body, Cr-Mn alloy steel powder has been hitherto used (Japanese Patent Publication No. 58-10962). Although Cr and Mn serve to increase hardenability and thus, have the merit of high strength after heat treatment, they are, respectively, ready-to-oxidize elements, with the attendant drawback that Cr--Mn composite oxide is formed to lower the fatigue strength and toughness of the resultant sintered body.
To avoid this, it is essential for the manufacture of Cr--Mn alloy sintered bodies to sinter and reduce in an atmosphere where an oxygen content is small and to use a specific type of vacuum reduction furnace.
The present applicant has already developed (Japanese Patent Laid-open No. 4-165002) a Cr alloy steel powder wherein the content of Mn is reduced and to which Nb and V are added. Since the Mn content is reduced, the severeness of the sintering atmosphere can be mitigated and the sintering may be effected not only in vacuum, but also in an atmosphere of N.sub.2 and/or H.sub.2. Accordingly, ordinarily employed sintering furnaces are sufficient for this purpose. However, according to the further investigations made by us, it has been found that the Cr-based alloy steel powder is disadvantageous in that the sintered body is increased in strength through the precipitation of carbides and/or nitrides of Nb and V, so that the fatigue strength and toughness lower owing to the existence of the carbides and nitrides which act as sites of fracture.
Where iron parts for which high strength is required are fabricated according to the powder metallurgical technique, it is usual to obtain necessary characteristics by a procedure which comprises sintering an alloy steel powder that is a mixture of pure iron powder and alloy element powders, or a green compact of the alloy steel powder and then subjecting to carburizing or nitriding treatment, followed by thermal treatments such as quenching and tempering. Accordingly, using the fabrication procedure, it is unavoidable to increase the fabrication costs and lower the dimensional accuracy owing to the thermal treatments.
To avoid this, Japanese Patent Laid-open No. 63-45348 discloses a technique wherein sintering activating powder and graphite powder are mixed with an alloy steel and the mixture is molded and preheated. Subsequently, the preheated mixture is sintered at 1140.degree.-1200.degree. C. and cooled at a cooling rate of 20.degree.-120.degree. C./minute to 200.degree. C. The method set out in the Japanese Patent Laid-open No. 63-45348 has the problem that since the sintering activating powder is mixed, the compressibility of a green compact lowers and that the structural uniformity of the sintered product is not high, with the sintered body having a varying dimensional accuracy.
Japanese Patent Laid-open No. 63-33541 proposes a method wherein an alloy steel powder whose contents of C, Si, P, S, N and O are reduced and to which Ni, Cr and Mo are added is sintered at 1100.degree.-1350.degree. C. and, after sintering, cooled at a cooling rate of 0.15.degree. C./second to obtain a sintered body having a strength not smaller than 110 kgf/mm.sup.2. However, since the alloy powder contains 3.0-4.5% of Cr, there arises the problems that oxides are liable to form, that the compressibility at the time of molding is poor and that the sintered body does not increase in strength.
As shown in the examples set forth in this application, the alloy steel powder inevitably contains 0.13-0.18% of Mn and P, S are present in amounts not smaller than 0.01%. The resultant sintered body has inconveniently low fatigue strength and toughness.
The invention has for its object the provision of alloy steel powders used to manufacture sintered bodies and also of sintered bodies obtained therefrom, which overcome the hitherto known problems involved by sintered bodies as set out hereinabove and which ensure sintered bodies having high strength, high fatigue strength and high toughness.
The invention also has as another object the provision of a method for manufacturing a high strength iron sintered body, as will not be obtained only by prior art sintering, in high dimensional accuracy and in a relatively inexpensive manner while omitting thermal treatments.